As integrated circuit device scaling continues, the central processing unit (CPU) current levels continue to increase due to several factors: an increase in the number of transistors per unit area on a die; introduction of new performance features; an increase in the number of cores in a processor; and reduction in supply voltage while the power envelope remains constant.
Among the deleterious impacts of increased current are the need to design a higher power voltage regulator and system power supply, the need for higher voltage to compensate for IR droop, an increase in the number of capacitors required and the need for better voltage regulators to supply higher current. Guard-banding is a technique that compensates for expected maximum current (Icc), degradation caused by factors such as excess current, and may involve degrading threshold voltage or increasing operating voltage (VDD) among other things.
In particular, at present, the maximum current for a CPU is determined by the worst case “power virus” and is handled as a given worst case. The term power virus generally refers to a carefully tuned computer program that executes specific machine code in order to reach the maximum CPU power dissipation (thermal energy output for the central processing unit). Computer cooling apparatus are designed to dissipate power up to the Thermal Design Power (TDP), rather than maximum power, and a power virus could cause the system to overheat; if it does not have logic to stop the processor, this may cause permanent damage.
The need to set the maximum current limitation to account for such problems as power virus-induced current increase may place severe constraints on operation of processors, such as multicore processors. Among present day approaches to address the constraints placed by the maximum current limitations efforts focused on:
Designing for worst case current
Designing the voltage regulator to “typical” worst case, adding statistical evaluation and not sum of worst cases of the entire system. Such a voltage regulator is equipped with over current protection that provides feedback on overload and eventually shuts down in critical conditions.
Increasing voltage guard-bands dynamically when high current is detected.
However, these approaches suffer from several problems. The approaches employ analog detection that results in a high degree of inaccuracy and requires calibration. These approaches also have difficulty meeting required bandwidth, and the response time for increasing guard-bands or responding to feedback may be to too long, thereby impacting performance. In worst case power virus scenarios, for example, current surges may develop within several nanoseconds. In systems that employ voltage regulators to implement a control loop, the response time is too slow to address such surges in a timely fashion.
It is with respect to these and other considerations that the present improvements have been needed.